The presence of scatter in projectional images acquired for tomographic reconstruction is currently a critical limiting factor in transmission CT modalities, such as medical x-ray Cone-Beam CT (CBCT), as it results in a significant degradation of the tomographic image quality and accuracy. In the field, the prominent methods that have been proposed so far for scatter correction include Monte Carlo methods, which are accurate but slow, analytical methods, which may lack the complexity to deal with heterogeneous objects (such as patients) accurately, and scatter suppression methods (such as the use of anti-scatter grids or the air gap method), the efficacy of which thus far have been limited. More recently, a proposed method included an experimental blocker method for cases where there exists repeat scans, such as in image guidance in radiotherapy, an initial partially blocked scan can be used to experimentally determine the patient specific scatter profile, and then through a process such as rigidly registration, subsequent scans can be corrected using the stored scatter profiles. Although this method has yielded promising results, it is specific to cases where there exists repeat scans, requires registration, and relies on the assumption that the patient configuration, such as positioning and weight, has not changed during the course of treatment.
Further, the x-ray scatter incurred to detector represents a bottleneck problem in volumetric image guided and adaptive radiation therapy. Several methods using a beam blocker for the estimation and subtraction of scatter have been proposed. However, due to missing information obstructed by the blocker, such methods require dual scanning or dynamically moving blocker to obtain a complete volumetric image.
CBCT systems mounted on the gantry of the linear accelerator have become an integral method for volumetric image guidance in modern radiotherapy. However, due to the broad beam geometry utilized in such systems, projection data are significantly convoluted by x-ray scatter, which in turn degrades the quality of CBCT, leading to a decrease in contrast, shading artifacts, and inaccuracies of CT number. Consequently, the utility of CBCT scans in important applications, such as adaptive radiotherapy, is limited. An efficient and practical scatter-corrected reconstruction technique is thus necessitated to maximize the usefulness of the on-board CBCT imaging system.
Many scatter correction strategies have been proposed in literature with recent research focused on beam blocker-based techniques. In these beam blocker-based approaches, the detected information in blocked regions is attributed to the scatter and scatter-free primary data are obtained by subtracting the estimated scatter data in blocked regions from the image data in unblocked regions. These methods can be divided into two major parts: a single scanning and a dual scanning. In the single scanning scheme, the missing information blocked by the lead strips is made available through interpolation. This interpolation may lead to the degradation of CBCT image quality due to unphysical smoothing and loss of geometric fidelity, especially in regions with sharp boundaries or edges. To get reasonable sampling rate in a single-rotation, a dynamically moving blocker-based approach has been introduced, but such a method necessitates an accurate synchronization between a finely motor-controlled blocker and a gantry rotation to avoid lag effects in the acquisition process. The dual scanning scheme has been attempted to avoid missing information caused by the beam blocker. A prior image based approach is to measure the scatter distribution with partially-blocked projection data taken at the initial day of treatment using a blocker. The estimated scatter distribution is used to correct later CBCT scans of the same patient. Though, this approach needs a minimal amount of extra imaging dose to generate a prior image, its accuracy depends on an image registration technique to correct geometric inaccuracy between prior and subsequent images because there are mismatched patient setup and intra-fractional organ variations. Another approach is to stitch up two projections with non-overlapped regions for complete projections by a dual-rotation CBCT scans. However, additional imaging dose and mechanical modifications of the current system are inevitable.
What is needed is a method of detecting CBCT source scatter data simultaneously with image projection data in a single scan, and correcting for scatter-induced artifacts in the CBCT image.